1. Field of the Invention
The present invention relates to an actuating device that is capable of stably occupying at least two discrete positions, and more particularly, to such a device used to move an aerodynamic or hydrodynamic surface.
2. Description of Related Art
Helicopters and tiltrotors are susceptible to a form of disturbance known as "1/rev vibration," so-called because it occurs at a frequency that matches the angular velocity of the helicopter or tiltrotor blades. There are numerous sources of 1/rev vibration, but a major source is non-uniformity in the aerodynamic contour, mass and stiffness distributions of individual blades. Blades with different aerodynamic, inertial and structural properties will generate different forces and moments, thus producing a once-per-revolution forcing of the helicopter or tiltrotor aircraft. These same forces will cause the blades to travel in different paths as they rotate. Thus, 1/rev vibration can be reduced if the blades are modified to all fly in the same path.
Theoretically, this problem should be solvable by closely controlling manufacturing tolerances of the blades. However, cost and practical constraints prevent perfectly matching individual blades in all aerodynamic, structural and inertial properties. Moreover, any repairs occasioned by, for example, damage from handling or in combat, would inevitably change a blade's mass and stiffness distributions.
A preferred manner of controlling 1/rev vibration mounts an individually adjustable tab on the trailing edge of the helicopter or tiltrotor blade. The aerodynamic properties of the blade are controlled by making preflight settings of the blade root pitch angles and trailing edge tabs on the blades. The positions of such tabs are adjusted to change the lift of each rotor blade so that all of the blades "track" (follow the same path in space) as they rotate. The tabs are typically made of plastically deformable metal. To adjust tracking, each individual blade's track is determined while the rotor is rotating. The rotor is then stopped and ground personnel change the tab position. The process is repeated until the blades track sufficiently well to reduce 1/rev vibrations to an acceptable level.
This approach has numerous drawbacks. For one thing, it requires specialized tooling to bend the tabs into their desired positions. It also requires highly skilled and specially trained personnel, and it is only through a great deal of experience with a particular aircraft that maximum efficiency is achieved in reducing 1/rev vibrations. Another drawback for some applications is that a metal tab reflects electromagnetic waves and therefore is incompatible with "stealth" requirements. If a non-metallic composite material is used for the tab, it must be temporarily softened by heating to allow repositioning, which introduces additional steps into the adjustment process.
There have been attempts at providing control surfaces capable of on-blade adjustment. These attempts use active piezoelectric, magnetostrictive or shape-memory alloy materials that can augment or replace mechanical rotor controls by using electrically actuated systems. U.S. Pat. Nos. 5,114,104, 5,150,864, 5,224,826, 5,366,176 and 5,662,294 disclose examples of those kinds of systems. Systems like the ones disclosed in these patents introduce a variety of complexities associated with the precise control required to maintain the desired control surface position. More significant problems include the requirement of some form of localized sensing of the control surface position and the need for constant application of electrical power to maintain the control surface in the desired position.
My U.S. Pat. No. 5,752,672 addresses problems in prior art approaches using shape-memory alloy actuators to change the position of control surfaces on rotating machinery such as a helicopter rotor or other inaccessible devices. In one embodiment of the invention described in detail, a helicopter rotor tab is plastically deformed by antagonistic shape-memory alloy actuating members. That invention eliminated the necessity of maintaining power to the actuating shape-memory alloy members.
However, it requires multiple actuating wires along the tab for a typical helicopter or tiltrotor tab extending for 15% of the blade span and requires a tab made of a plastically deformable material such as aluminum. As a result this system is incompatible with stealthy operation. It would be possible to use a composite material for the tab and stop the rotor to permit plastic deformation of the tab each time an adjustment is to be made. However, that would negate a principle advantage of the invention, which is permitting tab adjustment on a rotating helicopter blade. An additional problem is the necessity of ensuring that each wire bends the tab the same amount so that the tab is not deformed as its deflection is adjusted.
Giurgiutiu, Victor, et al., "Incrementally Adjustable Rotor-Blade Tracking Tab Using SMA Composites," American Institute of Aeronautics and Astronautics, Inc., Proc. 38th Structures, Structural Dynamics and Materials Conference, Kissimmee, Fla., Apr. 7-10, 1997, AIAA Paper No. 97-1387, pages 1456-1466, discloses shape-memory alloy wires embedded in multiple rotor tabs made from an elastic composite material. The tabs are deflected by actuating the wires while the rotor is rotating until tab positions are found that provide the least vibration. In one embodiment the actuator settings that minimized vibration while the rotor was in motion are duplicated after the rotor is stopped. The re-deflected tabs are then hardened by heating into the desired position. Such a system is inconvenient if the tab positions ever need to be readjusted.
The drawbacks of the above approaches apply equally when the goal is to move aerodynamic surfaces besides control tabs for helicopter and tiltrotor blades. An aerodynamic surface capable of moving between discrete positions is disclosed in Duffy, Robert E., et al., "A Theoretical and Experimental Study of the Snap-Through Airfoil and Its Potential as a Higher Harmonic Control Device," American Institute of Aeronautics and Astronautics, Inc., Proc. 26th Aerospace Sciences Meeting, Reno, Nev., Jan. 11-14, 1988, AIAA Paper No. 88-0668, pages 1-11. A large portion of the underside of a wing section is hollowed out and then covered with a skin that "snaps" in and out to change the camber of the wing. The skin is moved using a mechanical linkage and hydraulic actuator in the hollowed out portion of the wing section. It is immediately apparent that such an approach cannot be used to provide a movable tab on a rotor blade, and that the use of hydraulic actuation is cumbersome and heavy.
Of course, so-called "snap-through" actuators per se are known. Yang, Yao-Joe, et al., "Dynamics of a Bistable Snapping Microactuator," Proc. SPIE Smart Structures and Materials Conference, San Diego, Calif., February 1995, Vol. 2443, pages 754-762, discloses a silicon bistable snapping actuator with dimensions of 200 .mu.m.times.50 .mu.m. Obviously, such an actuator is unsuitable for moving an aerodynamic or hydrodynamic surface, that is, a surface on which act forces generated by moving the surface through a fluid.